But just what can a single state with a relatively small number of delegates up for grabs tell us about the rest of the primary season? If history is any guide, the Iowa caucuses really do matter.
The winner of the Iowa caucuses on the Democratic side has frequently gone on to be the Democratic nominee. Since 1972, there have been nine primary seasons without a Democratic incumbent president running. Six of nine times (67%), the Iowa winner was also the Democratic nominee. One off these non-successes (Tom Harkin in 1992) was from Iowa.
The success rate of Iowa winners does decrease, if you expand it out to include Republican caucuses as well. Including those, nine of 16 (56%) winners of the Iowa caucuses went on to win their party's nomination.
Remember: There have been more than 100 candidates who have participated in the caucuses since 1972. So something that tells us the winner over half the time is truly a value add.
A closer look at the polling indicates that Iowa isn't just correlated with success. It likely causes it both in the next contest (the New Hampshire primary) and nationally.
Winners of the Iowa caucuses have jumped a median of three percentage points in the New Hampshire primary polls following their win. Moreover, outperforming expectations (i.e. polling) in Iowa seems to have an additional effect. For every point that candidates outperform their Iowa polls, they get a bounce of 0.5 points in New Hampshire polls. Candidates who underperform their polls in Iowa see their New Hampshire poll number deflate by 0.5 points for every point they undershoot their New Hampshire polls.